2.1. Cancer
Cancer is characterized primarily by an increase in the number of abnormal cells derived from a given normal tissue, invasion of adjacent tissues by these abnormal cells, and lymphatic or blood-borne spread of malignant cells to regional lymph nodes and to distant sites (metastasis). Clinical data and molecular biologic studies indicate that cancer is a multistep process that begins with minor preneoplastic changes, which may under certain conditions progress to neoplasia.
Pre-malignant abnormal cell growth as exemplified by hyperplasia, metaplasia, and dysplasia (for review of such abnormal growth conditions, see Robbins and Angell, 1976, Basic Pathology, 2d Ed., W. B. Saunders Co., Philadelphia, pp. 68-79) precedes the formation of a neoplastic lesion. A neoplastic lesion may evolve clonally to grow into a solid tumor, and develop an increasing capacity for invasion, growth, metastasis, and heterogeneity, especially under conditions in which the neoplastic cells escape the host's immune surveillance (Roitt, I., Brostoff, J and Kale, D., 1993, Immunology, 3rd ed., Mosby, St. Louis, pps. 17.1-17.12).
Leukemia refers to malignant neoplasms of the blood-forming tissues. Transformation to malignancy typically occurs in a single cell through two or more steps with subsequent proliferation and clonal expansion. In some leukemias, specific chromosomal translocations have been identified with consistent leukemic cell morphology and special clinical features (e.g., translocations of 9 and 22 in chronic myelocytic leukemia, and of 15 and 17 in acute promyelocytic leukemia). Acute leukemias are predominantly undifferentiated cell populations and chronic leukemias more mature cell forms.
Acute leukemias are divided into lymphoblastic (ALL) and non-lymphoblastic (ANLL) types. They may be further subdivided by their morphologic and cytochemical appearance according to the French-American-British (FAB) classification or according to their type and degree of differentiation. The use of specific B- and T-cell and myeloid-antigen monoclonal antibodies are most helpful for classification. ALL is predominantly a childhood disease which is established by laboratory findings and bone marrow examination. ANLL, also known as acute myeloblastic leukemia (AML), occurs at all ages and is the more common acute leukemia among adults; it is the form usually associated with irradiation as a causative agent.
Chronic leukemias are described as being lymphocytic (CLL) or myelocytic (CML). CLL is characterized by the appearance of mature lymphocytes in blood, bone marrow, and lymphoid organs. The hallmark of CLL is sustained, absolute lymphocytosis (>5,000/μL) and an increase of lymphocytes in the bone marrow. Most CLL patients also have clonal expansion of lymphocytes with B-cell characteristics. CLL is a disease of older persons. In CML, the characteristic feature is the predominance of granulocytic cells of all stages of differentiation in blood, bone marrow, liver, spleen, and other organs. In the symptomatic patient at diagnosis the total WBC count is usually about 200,000/μL, but may reach 1,000,000/μL. CML is relatively easy to diagnose because of the presence of the Philadelphia chromosome.
The very nature of hematopoietic cancer necessitates using systemic chemotherapy as the primary treatment modality. Drugs selected according to sensitivities of specific leukemias are usually given in combination. Radiation therapy may be used as an adjunct to treat local accumulations of leukemic cells. Surgery is rarely indicated as a primary treatment modality, but may be used in managing some complications. Bone marrow transplantation from an HLA-matched sibling is sometimes indicated.
2.2. Arsenic and its Medical Uses
Arsenic has been considered to be both a poison and a drug for a long time in both Western and Chinese medical practices. In the latter part of the nineteenth century, arsenic was used frequently in attempts to treat diseases of the blood in the West. In 1878, it was reported that treatment of a leukemic patient with Fowler's solution (a solution containing potassium arsenite, valence +5) reduced markedly the count of white blood cells (Cutler and Bradford, Am. J. Ned. Sci., January 1878, 81-84). Further interests in the use of Fowler's solution as a palliative agent to treat chronic myelogenous leukemia (CML) was described by Forkner and Scott in 1931 (J. Am. Med. Assoc., 1931, iii, 97), and later confirmed by Stephens and Lawrence in 1936 (Ann. Intern. Med. 9, 1488-1502). However, while the active chemical ingredient(s) of Fowler's solution was not determined, its toxicity was well recognized. Fowler's solution was administered strictly as an oral composition, and was given to leukemic patients as a solution until the level of white blood cells was depressed to an acceptable level or until toxicities (such as skin keratoses and hyperpigmentation) developed, while the patients enjoyed varying periods of remission. In the 1960's, Fowler's solution was still used occasionally in attempts to treat CML, however, most patients with CML were treated with other chemotherapeutic agents, such as busulfan, and/or radiation therapy (Monfardini et al., Cancer, 1973, 31:492-501).
Paradoxically, one of the long recognized effects of exposure to arsenic, whether the source is environmental or medicinal, is skin cancer (Hutchinson, 1888, Trans. Path. Soc. Lond., 39:352; Neubauer, 1947, Br. J. Cancer, 1:192). There were even epidemiological data to suggest that the use of Fowler's solution over long periods could lead to an increased incidence of cancer at internal sites (Cuzick et al., Br. J. Cancer, 1982, 45:904-911; Kaspar et al., J. Am. Med. Assoc., 1984, 252:3407-3408). The carcinogenicity of arsenic has since been demonstrated by the fact that it can induce chromosomal aberration, gene amplification, sister chromatid exchanges and cellular transformation (See e.g., Lee et al., 1988, Science, 241:79-81; and Germolec et al., Toxicol. Applied Pharmacol., 1996, 141:308-318). Because of the known carcinogenic effect of arsenic, its only therapeutic use in human in Western medicine today is in the treatment of tropical diseases, such as African trypanosomiasis, (the organic arsenical, melarsoprol; See Goodman & Gilman's The Pharmacological Basis of Therapeutics, 9th edition, chapter 66, 1659-1662, 1997).
In traditional chinese medicine, arsenous acid or arsenic trioxide paste has been used to treat tooth marrow diseases, psoriasis, syphilis and rheumatosis (Chen et al., 1995, in Manual of Clinical Drugs, Shanghai., China, Shanghai Institute of Science and Technology, p. 830). In 1970's, arsenic trioxide had been applied experimentally to treat acute promyelocytic leukemia (APL) in China (commented by Mervis, 1996, Science, 273:578). The clinical efficacy of arsenic trioxide has recently been re-investigated in 14 of 15 patients with refractory APL, where the use of an intravenous dose at 10 mg/day for 4-9 weeks was reported to result in complete morphologic remission without associated bone marrow suppression (Shen et al., 1997, Blood, 89:3354-3360). It was also shown that arsenic trioxide induced apoptosis (programmed cell death) in vitro in NB4 cells, an APL cell line, and that apoptosis was apparently associated with down-regulation of the oncogene bcl-2, and intracellular redistribution of the chimeric PML/RARα protein that are unique to APL cells (Chen et al., 1996, Blood, 88:1052-1061; Andre et al., 1996, Exp. Cell Res. 229:253-260). It has been reported that the biological activity of arsenic is due to the ability of arsenic to direst the nucleoplasmic fraction of PML to nuclear bodies for degradation (Zhu et al., 1997, Proc. Natl. Acad. Sci., 94:3978-3983).
Although arsenic is well known to be both a poison and a carcinogenic agent, there have been many reports concerning the use of arsenic in medical treatment. Further, from the above discussion, it should be clear that there are many different types of leukemias, each of which requires a unique treatment protocol that is modified according to the presence of factors predicting for a risk of treatment failure. Thus, the development of a broad spectrum anti-leukemia agent that can be used alone or in combination with other existing drugs is extremely desirable.